1. Field of the Invention
The present invention generally relates to semiconductor devices, and, more particularly, to a semiconductor device provided with a durable metal film formed on an electrode pad. The present invention also relates to a method of producing such a semiconductor device.
2. Description of the Related Art
Conventionally, a semiconductor device has a metal film called a barrier metal layer formed on an electrode pad. Such a barrier metal layer may be formed by electroless metal plating. For instance, FIG. 1A is an enlarged schematic view of a part including a barrier metal layer of a conventional semiconductor device. A semiconductor device 100 has an electrode pad 103 on a chip 101 made mainly of silicon. A barrier metal layer 105 is disposed on the electrode pad 103. A passivation film 106 serving as a protection film is further disposed in an area where the barrier metal layer 105 does not exist.
As shown in FIG. 1A, the peripheral end portion 105A of the barrier metal layer 105 is in contact with the passivation film 106. Here, the peripheral end portion 105A has substantially the same thickness as the passivation film 106. Also, the barrier metal layer 105 is entirely flat.
FIG. 1B is an enlarged schematic view of a part including a barrier metal layer of another conventional semiconductor device 200. In FIG. 1B, the same components as in FIG. 1A are denoted by the same reference numerals. The barrier metal layer of FIG. 1B is slightly thickener than the barrier metal layer of FIG. 1A. Accordingly, the semiconductor device 200 differs from the semiconductor device 100 in having a slightly thicker barrier metal layer 115 and its peripheral end portion 115A. In FIG. 1B, the peripheral end portion 115A covers an end portion of the passivation film 106. However, the rest of the barrier metal layer 115 has a uniform thickness and an entirely flat shape.
The semiconductor devices 100 and 200 both have a solder bump 107 on the respective barrier metal layers 105 and 115 to be attached to external electrodes.
As described above, the semiconductor devices 100 and 200 are connected to external electrodes via solder bumps. Conventional solder bumps are made mainly of lead. In recent years, however, solder bumps made mainly of tin are preferred in consideration of the environment.
However, when solder bumps made of tin as a main component and silver are used, the strength of the peripheral end portions 105A and 115A in contact with the solder bump 107 decreases, thereby causing incomplete bonding. Because of such incomplete bonding, there is a risk of the electrode pad 103 and the solder bump 107 being brought into contact with each other and nullifying the function of the barrier metal layer 105. Also, there is another problem that the solder bump 107 and the barrier metal layer 105 might become separated from the electrode pad 103.
A general object of the present invention is to provide semiconductor devices in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a semiconductor device having a highly resistant metal film formed on an electrode pad, and a method of producing such a semiconductor device.
The reasons of the above problems are not known in detail. However, it is assumed that digestion and diffusion of a metallic element constituting a solder bump progresses preferentially at the peripheral end portion of a metal film. The inventors of the present invention have studied the above problems, and have discovered that the metal film should have a certain shape to reduce the occurrence of the problems.
The objects of the present invention are achieved by a semiconductor device comprising: an electrode pad; a metal film formed on the electrode pad; a protection film formed in an area where the metal film does not exist; and a bump disposed on the metal film. The metal film has a greater thickness at its peripheral end portion which is in contact with the protection film.
As the result of intensive studies made by the inventors, it was found that, even if the solder bump contains a component having digestion and diffusion properties, the thicker peripheral end portion of the metal film in contact with the protection film can improve the durability of the metal film. In the semiconductor device of the present invention, the peripheral end portion of the metal film is thicker than the flat portion of the metal film. When bonding the semiconductor to an external terminal via solder bumps, the peripheral end portion can presumably disperse the diffusive element, thereby improving the durability of the metal film.
The peripheral end portion of the metal film may be 1.3 to 2 times thicker than the flat portion of the metal film.
The metal film of the semiconductor device of the present invention may have a peripheral end portion covering the inner peripheral end portion of the protection film.
The objects of the present invention are also achieved by a semiconductor device production method including the steps of activating the surface of the electrode pad with a chelating solution containing glycine and a compound having a metallic element as nuclei, and forming a metal film by electroless metal plating.
By activating the surface of the electrode pad, the metallic element is precipitated on the surface of the electrode pad, which is a suitable condition for metal plating. Electroless metal plating is then performed on the surface of the electrode pad to form a metal film with the metallic element as nuclei. The peripheral end portion of the metal film in contact with the protection film is thicker than the central portion of the metal film.
The above and other objects and features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.